1. Field of the Invention
Embodiments of the present invention generally relate to a process and apparatus of forming a conductive layer containing metal fibers on a substrate.
2. Description of the Related Art
Many electronic devices, such as solar cells, LCD displays and touchscreen technologies use transparent conductive oxide (TCO) films as electrodes to provide a low-resistance electrical contact to a device's active layers. Transparent conductive oxide films are optically transparent and thus allow the passage of light through these conductive layers. In single- and tandem-junction thin-film silicon solar technologies, one or more of the electrical contacts are typically made from a deposited TCO layer that is connected to an active region of the solar cell device, such as the p-i-n junction. An example of a TCO containing thin film solar cell device 100 is shown in FIG. 1. In this example, the solar cell 100 may comprise a substrate 102 (e.g., glass substrate), a first transparent conducting oxide (TCO) layer 110 (e.g., zinc oxide (ZnO), tin oxide (SnO)), a first p-i-n junction 120, a second TCO layer 140 and a metal back contact layer 150, which when in use is oriented toward a light source 101 as shown. The first p-i-n junction 120 may comprise a p-type amorphous silicon layer 122, an intrinsic type amorphous silicon layer 124 formed over the p-type amorphous silicon layer 122, and an n-type amorphous silicon layer 126. In the p-i-n junction layer, incident sunlight (e.g., solar radiation 101) is absorbed and photogenerated electrons and holes are created, separated from one another, and finally transported to the opposite collection electrodes, such as TCO layer 110 and second TCO layer 140, and metal back contact layer 150, thus generating a photocurrent.
However, metal oxides used to form the TCO layers possess a number of disadvantages that reduce the absolute efficiency of a solar cell device, or affect the usability of other types of electronic devices. For example, one must balance optical transparency of the formed layer and its sheet resistance, where, for example, a thicker film or higher doping level in the metal oxide layer will lead to higher conductivities, but drastically reduce the ability of the film to transmit the incident light. For typical solar cells, this can translate to a photocurrent loss of 3 mA/cm2. Secondly, the low work-function of the metal oxide is often mismatched to the abutting active layer, which reduces the photovoltage in a device and requires the need for additional non-active corrective film layers that also absorb useful light. Finally, the cost associated with forming a TCO coating is quite expensive, such as between $7-$9/m2 of device area.
Thin-film solar, display, and touchscreen technologies are currently looking towards using metallic nanowires as a replacement for metal oxide films to replace the TCO layers. The metallic nanowires can be fabricated entirely from a solution and deposited onto a substrate using simple deposition methods. Current state-of-the-art electrically-conductive nanowire containing layers are based on random mesh networks deposited onto a substrate surface.
Random mesh networks of electrically-conductive nanowires can be deposited onto substrate surfaces using a variety of techniques and technologies. One such example of a simple and versatile method for fabricating ultrafine fibers with diameters ranging from nanometers to micrometers (i.e., nanofibers) is electrospinning. In a typical electrospinning procedure, a high voltage is applied to a metallic capillary, which is connected to a reservoir holding a polymer-containing solution with proper viscosity, conductivity, and surface tension. In some cases, this solution can be a composite, consisting of a number of various polymers and/or metal salts. The surface tension of the polymer solution can be overcome with a sufficiently high electrical field causing ejection of a thin fibrous jet of a metal-polymer material that can be collected onto a rigid or flexible substrate. Since the nanofibers are electrically insulating at this point, they must be further processed (e.g., metallicized) in order to conduct electricity.
The current state-of-the-art process of forming a conductive layer using an electrospinning process typically includes at least three separate processing steps. First, an electrically-insulating electrospun nanofiber, which comprises a metal salt and organic binding agent, is deposited on a surface of a collecting substrate. Next, the electrically-insulating nanofibers are converted to electrically conductive metallic nanofibers in two separate subsequent processing steps. In the first post electrospinning deposition step, the metal-polymer fibers are oxidized at high temperatures (˜500° C.) in order to remove the insulating polymer from the deposited metal salt and organic binding agent matrix. In the next post electrospinning deposition step, the now-converted metal oxide fibers are then “metallicized,” or the conductivity of the formed nanowires is improved, by placing them in contact with a reducing gas while maintaining the wires at about 250° C. for greater than about 2 hours, which is very time consuming, and as will be discussed below, not effective in metalizing the fibers. While these processes have proven to be adequate in producing metallic fibers, there are a number of inherent disadvantages with the current state-of-the-art processing techniques. For example, the current-state-of-the-art processes restrict one to hard, rigid collecting substrates only, in order to withstand the high temperatures associated with the oxidation process. One cannot directly deposit and process the electrospun electrically-insulating nanofibers directly onto a thin, flexible and/or polymeric or organic material containing substrate and then perform the nanofiber conversion processes, due to the risk of melting or damaging the substrate or layers formed thereon. Therefore, in order to form the nanowires on a flexible substrate, or substrate containing polymeric or organic material containing materials, the electrically-insulating electrospun nanofibers must first be spun onto a rigid high temperature resistant substrate (e.g., glass) and then be transferred after processing to a desirable flexible or polymeric material containing substrate, which adds another post processing step (e.g., 4th processing step). These conductive layer forming process steps thus make the process of forming next generation solar cells, LCD displays and touchscreen devices at a low cost challenging, if not impossible.
Therefore, there is a need for an apparatus and method of efficiently forming an electrospun conductive layer on a surface of a substrate that does not contain the drawbacks discussed above.